Polyoma virus and its polymorphic aggregates will be studied by low-irradiation electron microscopy and image analysis to answer questions about their overall structure, assembly and stability. Polyoma, a small DNA-containing virus, closely related to SV-40, causes transformation of embryonic or immunologically deficient mouse cells. We have recently discovered (Rayment, Baker, Caspar and Murakami (1982) Nature 295, 110-115) that the virus capsid is composed of 72 pentameric capsomeres rather than 12 pentamers and 60 hexamers as predicted by the Caspar-Klug quasi-equivalence theory of subunit assembly. This unexpected result prompts us to examine the packing and bonding specificity of the capsomeres in several polymorphic aggregates of polyoma using low-irradiation electron microscopy and image analysis. These methods are the most powerful way to study biological aggregates which have not been crystallized in a form suitable for X-ray diffraction. The molecular weights of intact virions and capsids will be determined using scanning transmission electron microscopy to provide an independent measure of the number of copies of VP1, the major coat protein. Electron microscopy studies of positive and negatively-stained provirion complex samples will be initiated to directly observe the nucleocapsid structure and correlate with studies on intact virions, empty capsids and isolated minichromosomes.